Continental Rise off Eastern North America

Abstract

During mid-1967 two cruises of the Woods Hole Oceanographic Institution's R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.